Positive displacement pumps, specifically vane-type positive displacement pumps, have found uses in the fuel systems of gas turbine engines. Typically, these vane pumps include a slotted rotor configured to accept closely fitted but free moving vanes. The rotor may be splined to accept a splined pump drive shaft. In some vane pumps, a lobe shaped cam ring surrounding the rotor defines at least one pumping chamber. Pressure plates may be positioned on either side of the cam ring/rotor assembly. Typically, the pressure plates include flow passages (i.e., inlets and outlets) for fluid entering and leaving the pumping chamber.
The pumping cycle is started when the rotor turns as the drive shaft is rotated. The centrifugal force acting on the vanes causes them to slide outward, or extend, in the rotor vane slots until they contact the contoured cam ring. As the rotor turns, the vanes “track” against the contour of the cam ring. During the intake portion of the cycle, when fluid is drawn into the pumping chamber, the clearance between the rotor and the cam ring increases and fluid is taken in to fill the spaces between the vanes left by the rising cam. This is also known as the intake cycle.
At the point where the vanes reach the maximum extension, the cam blends into the major diameter. The vanes, after passing through the major dwell portion of the cam, begin to retract on the descending cam contour. As the space between the cam ring and the rotor decreases, fluid is forced out of the spaces between the vanes by the falling cam contour. This is also known as the discharge cycle.
The displacement of dual-lobe cam ring vane pumps can be varied by rotating the cam ring. However, one problem associated with dual-lobe variable displacement pumps is pressure pulsation in the transition region. This may occur in the transition from inlet to discharge when there is insufficient discharge area relative to the compression rate, thus producing a rapid increase in pressure. Another problem with dual-lobe variable displacement pumps is cavitation in the transition region from discharge to inlet. This may occur when there is insufficient fill area relative to the volume expansion rate, thus producing a rapid decrease in pressure. Both of these problems expose the pump components to severe mechanical stresses which can reduce the reliability and the lifetime of the pump.
It would therefore be desirable to have a variable displacement pump, with a dual-lobe cam ring, that reduces or eliminates cavitation and pressure pulsation during pump operation.
Embodiments of the invention reduce or eliminate the aforementioned cavitation and pressure pulsation. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.